As the need for petroleum products developed over the years, wells were typically drilled and produced through employment of land based operations. Petroleum exploration as continued on land until, at this point, most major oil finds have been identified and production therefrom has been initiated. Obviously many small oil and gas reservoirs have not yet been located but, in time will be identified and produced to the extent that they are cost effective.
In its effort to locate significant reservoirs of petroleum products, the petroleum industry began prospecting in the marine environment a number of years ago. Many significant petroleum reservoirs have been identified and offshore drilling and production has likewise been developed to the extent that much of the worlds oil production now comes from reservoir beneath the sea. In order to locate, drill and produce subsea petroleum reserves, the equipment and procedures for the same become extraordinarily expensive. For example, at the cost of many millions of dollars, an offshore platform is designed, constructed and installed. If the platform is a drilling platform typically a number of wells are drilled from a single platform thereby amortizing its cost per well. A platform of this nature is typically a permanent installation and therefore only becomes cost effective in the event the petroleum reservoir is of significantly great volume that the effective service life of the platform will be in the range of from 20 to 40 years. If the petroleum reservoir is of small nature, though it may contain a significant volume of petroleum products, nevertheless it is not cost effective and therefore further exploration, drilling and production activities will be deferred until such time as increased oil prices renders drilling completion and production activities cost effective. It is desirable, therefore, to provide a system for drilling, completion and production of petroleum products which renders small petroleum reserves cost effective. This is accomplished by providing a platform system which may be temporarily located at the site of a small petroleum reserve. Multiple wells may then be drilled and produced utilizing the mobile platform system. After the petroleum reserve has been sufficiently depleted that further production is not cost effective, the wells can be shut-in and the platform apparatus moved to the marine site of another small petroleum reserve. Since the apparatus provides a self contained storage facility, it is not necessary to lay expensive offshore pipeline systems to conduct petroleum products to a remotely located gathering facility. Petroleum products recovered from the offshore reservoir may be loaded directly from the facility onto tankers or other appropriate petroleum transportation vessels for transportation to an appropriate storage and refining system.
Submersible and semisubmersible production and storage facilities as well as floating petroleum service vessels must be maintained at rather precise locations relative to the ocean floor to permit the specialized operations that are involved. In case of drilling precise stationing of the drilling rig is necessary to maintain the rotating drill stem as straight as possible as it is being rotated by the drilling rig. Especially in foul weather conditions and rather deep water locations, the vessels and structures are likely to be laterally displaced by wave action, wind and current. In cases where conventional mooring is accomplished by means of mooring cables or chains, the forces of wind current and wave action can cause tightening of the cables on one side of the vessel or structure and consequent loosening of the opposite cables, thus allowing the vessel or structure to shift laterally. The amount of lateral shifting or excursion that occurs depends on the forces applied to the vessel and to the curvature present in the mooring cables or chains. For example, a conventionally moored floating or semisubmersible vessel in 600 to 1,000 feet of water will be capable of lateral excursion in the order of 45 feet because of the catenary (curvatures) defined in the mooring cables. However, drilling activities can take place only when the structural or vessel is maintained within prescribed limits of lateral excursion. Drilling activities can therefore be conducted only when the drilling vessel misalignment above the wellbore is maintained to within about 2% of water depth. Under circumstances where wind, wave action and current cause the vessel or structure to shift laterally beyond the maximum prescribed for drilling, such drilling operations must cease. When drilling or servicing operations are being conducted in marine environments where stormy conditions frequently occur, for example in the North Atlantic and North Sea areas, drilling rigs are required to shut down quite frequently simply because the wind conditions, wave action and currents cause lateral excursion of the drilling rig beyond acceptable limits. This, of course, is detrimental to the cost of drilling operations because the fixed cost of maintaining the vessel, equipment and personnel continues during such periods of inactivity. It is of course desirable to provide a mooring system for floating and semisubmersible drilling rigs which will significantly reduce the amount of down time that is presently due to adverse weather conditions.
It is desirable, therefore, to provide a mooring system having no catenaries, thus efficiently minimizing lateral excursion in response to forces generated by wave action, wind and current and maintain the structure stable within allowable limits. In the case of submersible systems, obviously wind is not a factor and wave action may be only a minimal factor if the depth of location is well beyond significant influence of wave action. In such cases, the major force requiring resistance for efficient stationing of submersible apparatus is the force of current. Even in placid sea conditions, currents can run from less than one knot to a speed in the order of five knots under which directional variation of lateral excursion can be a significant factor. It is desirable, therefore, to provide a mooring system for submersible and semisubmersible petroleum storage systems and service vessels which is substantially catenary free, thereby minimizing lateral excursion of the system or vessel.